The present invention relates to a radio communication system employing orthogonal signal transmission techniques for the transmission of packets of data. The present invention also relates to a transmitter for use in such a system to a method of operating such a system and to a signal transmitted in such a system. While the present specification describes a system employing Orthogonal Frequency Domain Multiplexing (OFDM), it is to be understood that such techniques are equally applicable to other systems transmitting orthogonal signals, for example Code Division Multiple Access (CDMA).
OFDM, also known as MultiCarrier Modulation (MCM) or Discrete MultiTone modulation (DMT), is a technique by which data is transmitted at a high rate by modulating several low bit rate carriers in parallel, rather than one high bit rate carrier. OFDM is spectrally efficient, and has been shown to be effective for high performance digital radio links. Application areas include: Wireless Asynchronous Transfer Mode (WATM), for high speed, short distance radio links between computer systems; Digital Audio Broadcasting (DAB), for high quality audio signals; Microwave Video Distribution System (MVDS); and future mobile radio systems such as Universal Mobile Telecommunication System (UMTS).
An important characteristic of a Radio Frequency (RF) signal for transmission is the crest factor, defined as the ratio of the peak value of an AC waveform to its Root Mean Square (RMS) value. In an OFDM system the crest factor can be high since it is possible for the signals on each of the carriers to be in phase (giving rise to a peak value that is the product of the number of carriers and the amplitude of the signal on each carrier), but on average the phases will be randomly distributed (giving rise to a much lower mean value). For example, in a 16 carrier OFDM system the peak power can be 16 times the mean transmission power.
If such signals are to be transmitted without distortion, a high specification transmitter is required with good linearity. Generally such an transmitter has a poor DC to RF power conversion efficiency which may result in the generation of excessive amounts of heat and which also has a detrimental effect on battery life if the transmitter is incorporated in portable equipment. Various approaches have therefore been investigated for reducing the crest factor.
One technique is to prevent the combination of certain phase modulation states from being applied to the carriers. However, this has the disadvantage that more symbols need to be transmitted for a given amount of data as each symbol has fewer available states. Such techniques are well known, one example being a xc2xe rate scheme for a four carrier OFDM system, which reduces the crest factor from 4 to 1.9. U.S. Pat. No. 5,636,247 describes a more sophisticated technique of this type. When applied to a 16 carrier system a crest factor reduction of 3 dB can be achieved using a {fraction (13/16)} rate scheme.
An alternative method is described in U.S. Pat. No. 5,610,908, in which a number of closely spaced carriers are modulated (in this case using QPSK) and then transformed to the time domain by an Inverse Fast Fourier Transform (IFFT), as is usual. The signals are then limited and transformed back to the frequency domain by a Fast Fourier Transform (FFT) where phase and amplitude adjustments may be made to some of the signals, and then transformed back to the time domain with an IFFT. From here the transmission proceeds as normal. An example is given of a 2048 carrier OFDM system for which a simulation of twenty random signals, initially having a crest factor of 9.38 dB, demonstrated that the crest factor could be reduced to 3.4 dB.
It can be seen that although the techniques outlined above can reduce the crest factor they cannot reduce it to unity (corresponding to a constant envelope modulation). An alternative known technique for reducing the crest factor is clipping, where the baseband signal is amplitude clipped at a constant level, therefore removing signal peaks and reducing the crest factor. Clipping is a simple technique to implement, although because it is a nonlinear process some care is required in its use.
The effect of clipping in a 128 carrier OFDM system is discussed in the paper xe2x80x9cEffects of Clipping and Filtering on the Performance of OFDMxe2x80x9d by X Li and L J Cimini, Proceedings of the 47th IEEE Vehicular Technology Conference, May 1997, pp. 1634-1638. In this paper it is shown that setting a clipping level at about 1.5 times the mean power level provides a substantial reduction in crest factor without a substantial increase in bit error rate.
A problem with the use of clipping, not addressed in the prior art, is that certain OFDM symbols are adversely affected by it whereas others are unaffected. If a number of adversely affected symbols are transmitted in a packet the receiver is likely to fail to demodulate the packet and request its retransmission. The sender will repeat the packet and encounter the same problem. Hence, certain packets are very unlikely to be received without error.
An object of the present invention is to alleviate the problem that certain packets are very difficult to transmit.
According to a first aspect of the present invention there is provided a method of operating a radio communication system comprising encoding data onto a plurality of orthogonal carriers by differential phase modulation, combining the phase modulated signals, clipping the combined signal to limit the crest factor and transmitting the clipped signal as data packets between at least two stations, characterised by randomising the initial phases of at least one, of the carriers before transmission of a packet.
According to a second aspect of the present invention there is provided a transmitter for transmitting orthogonal signals, comprising differential phase modulation means for modulating data onto a plurality of orthogonal carriers, means for combining the phase modulated signals, clipping means for limiting the crest factor of the combined signal and transmission means for transmitting the clipped signal, characterised in that means are provided for randomising the initial phases of at least one of the carriers before transmission of a packet.
According to a third aspect of the present invention there is provided a radio communication system comprising a plurality of transmitters made in accordance with the present invention.
According to a fourth aspect of the present invention there is provided a radio signal comprising a plurality of orthogonal carriers onto which packets of data are encoded by differential phase modulation, the signal being clipped to limit its crest factor, characterised in that the initial phase of at least one of the carriers is randomised at the start of a data packet.
The present invention is based upon the recognition, not present in the prior art, that by varying the initial phase states of the carriers comprising an OFDM signal, repetition of a symbol will result in a different crest factor.